Abstract: Mud crab,Scylla serrata(Forskal),is main economically cultivated marine crab in China,with production of 108,500 tons,88.14%of the total production of the world.The main mud crab region distributed along the coastline of Southeast China,including Zhejiang,Fujian,Guangdong,Guangxi and Hainan Province.Recently,serious diseases occurred in main mud crab culture region,with the morbidity of 13.77% in cultivated mud crab in 2006 in whole China,resulting in 32.71 million dollars lost.White spot syndrome virus(WSSV),the most pathogen of shrimp culture in China,was considered an important pathogen in cultivated mud crab.It showed that WSSV detection with PCR was closely relative with occurring of mud crabs diseases during the epidemics investigations from 2006 to 2007.To further study the pathogenicity of WSSV to mud crab,WSSV bacteria-free suspension,collected from typical WSSV infected black tiger shrimp(Penaeus monodon),was used for challenging to healthy mud crab by intramuscular injection at 10-folded dilutions ranged from 1∶10 to 1∶10000.The results showed that the cumulative mortalities for dilutions of 1∶10 to 1∶100,1∶1000,1∶10000 were 100%,66.7% and 38.9% respectively.The LD50 of WSSV to mud crab was calculated as 1.19×106 copies/crab,or 7.93×103 copies/g of crab weight by Muench-Reed methods.Gills and hemolymph of moribund mud crabs challenged with WSSV were positive by PCR.These results indicated that WSSV was highly virulent to mud crab.The activities of main hemolymph enzymes,including phenoloxidase(PO),peroxidase(POD),superoxide dismutase(SOD),alkaline phosphatase(ALP),glutamate-pyruvate transaminase(GPT) and glutamic-oxaloacetic transaminase(GOT) of mud crab were analyzed.WSSV infected moribund crabs showed significantly descent in activities of PO,POD and SOD,while ascent in ALP,GPT and GOT compared with the control group.WSSV mainly distributed in epithelia of gills,cuticular of skeletal plate,heart,intestine and stomach by immunohistochemistry with Mabs against WSSV,and gill epithelia showed heavily damaged with the main distribution of WSSV in nucleic.It can be concluded that WSSV was a main pathogen of mud crab with high pathogenicity.